1. Field of the Invention
The present invention relates to a light-emitting semiconductor diode (LED) and a laser diode (LD) that use a Group III nitride compound and that has a multiple emission layer. Especially, the invention relates to an LED and an LD having an improved emission efficiency in the visible short wavelength region from the green blue to blue color and in the ultra violet region.
2. Description of the Related Art
It has been known that an aluminum gallium indium nitride (AlGaInN) compound semiconductor may be used to obtain a light-emitting diode (LED) or a laser diode (LD) which emits blue and ultra violet color light. This semiconductor device is useful because of its high luminous efficiency resulting from direct electron transition and because of its ability to emit blue light, which is one of the three primary colors.
By irradiating an electron beam and carrying out heat treatment, a magnesium (Mg) doped i-layer is changed into a p-type conductive layer. As a result, an LED or an LD is obtained having a double hetero p-n junction structure made of an aluminum gallium nitride (AlGaN) p-layer, a zinc (Zn) doped indium gallium nitride (InGaN) emission layer, and an AlGaN n-layer. Such an LED or LD is more prosperous in the semiconductor market than a conventional LED having a metal insulator semiconductor (MIS) structure which includes an n-layer and a semi-insulating i-layer.
As shown in FIG. 6, an LED 10 is disclosed in Japanese Patent Application No. 113484/1994 (not yet laid open) which has higher luminous emission. A GaInN emission layer 5 of the LED 10 is doped with both zinc (Zn) and silicon (Si), and both planes of the emission layer 5 form a double hetero-junction structure with an adjacent AlGaN n-layer 4 and an AlGaN p-layer 61. The peak wavelength of the LED 10 is between 420 and 450 nm and its luminous intensity is 1000 mcd. Such LEDs with higher luminous intensity of blue color are in great demand, for example, for use in multicolor display devices.
While light, having a peak wavelength of about 500 nm, resulting in a green blue or dark green color, is required for traffic signal lights, the conventional LED 10 is unable to provide that required wavelength. In order to meet that requirement, the energy band width of the emission layer needs to be narrowed by increasing the ratio of indium (In) among the components of the emission layer 5. Furthermore, both an acceptor impurity and a donor impurity are doped into the emission layer 5 controlling their impurity concentrations to maximize luminous intensity of the LED.
Such arrangements, increasing the In ratio among the composites of the emission layer 5 and doping the emission layer with an acceptor and a donor impurity, however, rather raise significant potential energy caused by the Coulomb force between the acceptor and the donor, and the electron transition energy becomes equal to the sum of the potential energy and the energy difference between the acceptor and donor levels. The energy difference between the acceptor and donor levels virtually becomes larger than that in the case of no Coulomb force. As a result, the peak wavelength is shifted toward a shorter wavelength in the luminous spectrum and the required wavelength peak of 500 nm cannot be obtained.
As shown in FIG. 10, a gallium nitride compound semiconductor device 20 with a multiple emission layer structure is disclosed in laid-open Japanese Patent Application number 268257/1994. The emission layer is formed by three In0.2Ga0.8N 44 wells and two In0.04Ga0.96N 44xe2x80x2 barriers. Each of them has a thickness ranging from 5 to 50 xc3x85 and they are laminated alternately.
The peak wavelength of the device 20 of FIG. 10 is still around 410 to 420 nm, because the emission mechanism of the device 20 is an inter-band recombination formed without doping any impurities in the wells 44 which act as a luminous center. Such a peak wavelength does not meet the required 500 nm wavelength for a traffic signal. Further, the luminous intensity of the device 20 still has room for improvement. Therefore, there is a need for an LED having both a larger peak wavelength and higher luminous intensity.
InGaN and AlGaN are representative materials for an emission layer of a Group III nitride compound semiconductor device which emits ultraviolet rays. When InGaN is utilized for the emission layer and the composition ratio of In is 5.5% or less an ultraviolet ray having a peak wavelength of 380 nm is obtained and the emission mechanism of the device is the inter-band recombination. When AlGaN is utilized for the emission layer, the emission layer is doped with Zn and Si, and the composition ratio of Al is around 16%, an ultraviolet ray having a peak wavelength of 380 nm is obtained and the emission mechanism of the device is the electron transition between energy levels of the donor and the acceptor.
Although the peak wavelength of such devices utilizing InGaN or AlGaN is satisfactory, the luminous efficiency of the same is still poor for several reasons. The emission layer made of InGaN has a poor luminous efficiency due to poor crystallinity as a result of low growth temperature and carrier recombination between bands. The emission layer made of AlGaN has a poor luminous efficiency due to a dislocation resulting in a mismatch of lattice constants.
A first object of the present invention is, therefore, to improve the luminous efficiency for blue color produced by an LED utilizing a group III nitrogen compound and to shift (lengthen) the peak wavelength of such an LED toward around 500 nm.
A second object of the present invention is to improve the luminous efficiency of ultra violet light produced by an LED or an LD utilizing a group III nitrogen compound.
In accordance with first aspect of the invention, a multiple emission layer is provided. Acceptor and donor impurities are alternately doped into each composite layer of the multiple emission layer so as to widen distance between the atoms of the acceptor impurity and the donor impurity.
In accordance with a second aspect of the invention, an undoped layer is provided between a donor doped layer and an acceptor doped layer so as to widen distance between the atoms of the acceptor and the donor impurity.
Conventionally, both a donor impurity and an acceptor impurity are doped into a single emission layer to obtain a higher luminous intensity. However, with an LED having such a structure, it is difficult to control the peak wavelength, and it is especially difficult to increase the length of the peak wavelength. The inventors of the present invention have performed research and have discovered that a close distance between the atoms of an acceptor impurity and the atoms of a donor impurity generates a Coulomb force which influences a transitional electron and substantially widens the energy level difference between the impurities. As a result, a longer peak wavelength cannot be obtained.
The emission peak energy h is calculated by:
h=Egxe2x88x92(ED+EA)+(q2/r)
when h is the Plank""s constant,  is the frequency of light, Eg is the energy band gap, ED is the activation energy of the donor, EA is the activation energy of the acceptor, r is the distance between atoms of the donor impurity and the acceptor impurity, q is the elementary electric charge, and is the dielectric constant.
As the expression shows, a longer peak wavelength is attained by a larger value r, or by a longer distance between the atoms of the acceptor impurity and the donor impurity. The inventors of the present invention propose several structural arrangements to obtain a larger value r. Namely, an emission layer is formed as a multi-layer structure, and its composite layers are alternately doped with an acceptor impurity and a donor impurity. Further, the thickness and/or composition ratio of the impurity-doped composite layers can be varied to obtain a desired peak wavelength. As further alternate, an undoped layer can be formed between impurity doped layers, and further modulation doping such as xcex4 doping can be used to dope impurities slightly into the composite layers.
With these arrangements, the distance r is widened. Since the Coulomb force corresponds to the distance from the center of a layer to that of another on average, the influence of the Coulomb force generated by the atoms of the acceptor and the donor impurity may be mitigated substantially by widening the distance r.
Consequently, the peak wavelength can be shifted toward longer value as intended, i.e., from 450 nm to 500 nm. In addition, other conditions of the emission layer, such as composite materials, their composition rate, doped impurities, their concentration and so forth, can be adopted and optimized to obtain maximum luminous intensity, so that high luminous intensity of blue color around 3000 mcd can be maintained.
In accordance with a third aspect of the invention, an emission layer is provided which has a quantum well (QW) structure with at least one set of a well and a barrier is doped with both an acceptor impurity and a donor impurity.
The QW structure contributes to an increase in output power and luminous intensity, because the well of the QW structure is surrounded by barriers each having a band gap wider than that of the well, and because carriers that emerge from the barriers are poured into the well and contribute to luminous emission. Consequently, luminous intensity is improved. Further, doping both an acceptor impurity and a donor impurity into the well lengthen the peak wavelength, because of the transition between energy levels of the acceptor impurity and the donor impurity, and improves the luminous intensity, because of abundant existence of carriers. Further, the acceptor impurity and the donor impurity may be doped into both the well and the barrier to obtain higher luminous intensity.
In accordance with a fourth aspect of the invention, an emission layer is provided which has a QW structure at least constituted by a set of an Alx2Ga1xe2x88x92x2N barrier and an Alx1Ga1xe2x88x92x1N well, where x1 less than x2.
The molar composition rate of Al is designed to be 15% or more in order to obtain shorter peak wavelength around 380 nm. The thickness of the well is designed to range from 50 xc3x85 to 200 xc3x85. It is preferred that the thickness of the well not be thinner than 50 xc3x85, because impurities are spread or diffused into an adjacent layer. It is preferred that the thickness of-the well not be more than 200 xc3x85, because a quantum effect cannot be expected. The thickness of the barriers is designed to range from 50 xc3x85 to 200 xc3x85. It is preferred that the thickness of the barriers not be less than 50 xc3x85, because the efficiency of carrier containment in the well drops. It is preferred that the thickness of the barriers not be more than 200 xc3x85, because a quantum effect cannot be expected. Further, barriers thicker than 200 xc3x85 are not preferred, because the barrier has a large resistivity when it is undoped, and may have cracks because of dislocations when it is doped with impurities.
The preferable impurity concentration of the acceptor impurity and the donor impurity doped into the QW emission layer is in the range from 1xc3x971017/cm3 to 1xc3x971020/cm3 respectively. It is preferred that the concentration of each impurity not be lower than 1xc3x971017/cm3, because luminous efficiency drops due to lack of luminous centers. It is preferred that their concentration of each impurity not be higher than 1xc3x971020/cm3, because crystallinity becomes poor and an Auger effect emerges.
The luminous efficiency is improved by utilizing AlGaN for the emission layer which has a better crystallinity than InGaN. And also the emission layer is constituted by a super lattice structure of QW preventing a mismatched lattice constant from spreading. Consequently, the crystallinity of the well and the luminous efficiency are improved. Further, a donor-acceptor pair emission layer formed by doping both an acceptor impurity and a donor impurity into the well or both the well and the barrier improves luminous efficiency.
In accordance with a fifth aspect of the invention, a QW emission layer is provided which comprises at least one set of a Alx1Gay1In1xe2x88x92x1xe2x88x92y1N well and a Alx2Gay2In1xe2x88x92x2xe2x88x92y2N barrier whose forbidden band is wider than that of the well. The well or both the well and the barrier are doped with either a donor impurity or an acceptor impurity. Accordingly, either a donor or an acceptor energy level is formed in the well or both in the well and the barrier, so that the possibility of recombination between electrons and holes due to the formation of a donor or an acceptor energy level increases. Consequently, luminous efficiency is greatly improved. Further, the composition ratio and the impurity concentration of indium (In) are optimized balancing intended peak wavelength and luminous intensity.
Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of the specification, wherein like reference numerals designate corresponding parts in the various figures.